The present invention relates to improved microcapsules and methods for making the same. More specifically, the improved microcapsules each comprise a core of fill material, at least one generally continuous wall encapsulating the core and the reaction product of a methylolated epoxy compound and material of the outermost wall as at least a portion of the outermost surface of the microcapsule. A method of making the improved microcapsules of the present invention comprises providing microcapsules including a core of fill material and at least one wall encapsulating the core and then reacting a methylolated epoxy emulsion with the outer surfaces of the wall of the microcapsule. The resultant microcapsules have improved properties making them adaptable for a variety of applications, particularly for use in carbonless copying systems.
Microcapsules generally comprise a core of fill material, typically having a size in the range of microns, surrounded by a wall or shell of polymeric type material. The fill material may be either gaseous, liquid or solid and may be composed of a single substance, a solution or a mixture. The wall surrounding the core of fill material typically functions to isolate the fill material from an external environment until it is desired to have the fill material available at which time the capsule wall is ruptured or removed in some fashion such as by mechanical pressure, dissolution or the like. Certain types of microcapsules are intended to remain intact without removal or rupture of the capsule wall such as microspheres, microbeads and the like.
Uses for microcapsules are almost as varied as the materials that may be microencapsulated. Of particular importance are the use of microcapsules, in medicinal preparations, fertilizers, flavorings, detergents, laundry brighteners, perfumes, disinfectants, deodorizers, adhesives, carbonless copying systems and the like.
The patent literature is replete with methods or techniques for preparing microcapsules and microcapsular products. A great majority of these methods generally comprise providing a dispersed phase of minute discrete droplets containing the intended fill material in a continuous phase and then forming encapsulating walls or shells about the minute droplets. Specific methods for forming the encapsulating walls about the droplets include simple and complex coacervation, interfacial polymerization, polycondensation and the like. Specific patents relating to microencapsulation methods include U.S. Pat. No. 3,429,827 to Ruus, relating to an interfacial polycondensation type method, U.S. Pat. No. 3,578,605 to Baxter, relating to an acid chloride-gelatin-gum arabic dual-wall type method and U.S. Pat. No. 3,016,308 to Macaulay, relating to a urea-formaldehyde condensation type method, among others.
While microcapsules and microencapsulation techniques are applicable to a wide variety of products, perhaps one of the most significant applications is in their use in carbonless copying systems. While the present invention is particularly adaptable to carbonless copying systems and will be discussed primarily hereinafter with regard to such systems, it should be understood that the invention is not thereby so limited and may be used in any application where the improved properties of the microcapsules are beneficial.
As is generally known in the art, carbonless copying systems generally comprise a plurality of paper sheets or webs arranged in a manifold set, each sheet or web of the set having one or more coatings on the surfaces thereof. The manifold set is designed so that when a marking pressure caused by a means such as a typewriter, pen, stylus or the like is applied to the outermost sheet, a colored mark will be formed on at least one surface of each sheet of the manifold set.
To this end, the top sheet of the manifold set to which the marking pressure is applied is provided with a coating on its back surface, the coating including microcapsules containing an initially colorless chemically reactive color forming dye precursor as the fill material. The upper surface of the sheet contiguous to the back surface of the top sheet is coated with a material containing a component capable of reacting with the colorless dye precursor contained in the microcapsules to produce a color. Thus a marking pressure on the upper surface of the top sheet will rupture the microcapsules on the bottom surface and thereby release the colorless dye precursor. The colorless dye precursor, upon contact with the reactive component of the coating of the lower sheet, will chemically react to produce a colored mark corresponding to the area of marking pressure. In a like fashion, colored marks are produced on each succeeding sheet of the manifold set by the marking pressure rupturing microcapsules carried on the lower surfaces of each sheet.
The sheets of the manifold set in carbonless copying systems are designated in the art by the terms CB, CFB, and CF which stand respectively for "coated back", "coated front and back", and "coated front". The CB sheet is usually the top sheet of the manifold set and the sheet upon which the marking pressure is applied; the CFB sheets are the intermediate sheets of the manifold set, each of which is able to have a mark formed on its front surface by a marking pressure and each of which also transmits the contents of ruptured microcapsules from its back surface to the front surface of the next succeeding sheet; and the CF sheet is the bottom sheet and is only coated on its front surface so that an image may be formed thereon.
While it is generally customary to have the coating containing the microcapsules on the back surface of the sheets and to have the coating containing the reactive component for the capsule content on the front surface of each of the sheets, a reverse arrangement may also be utilized. In addition, in some systems the coatings need not be used at all and the reactive ingredients may be carried in the sheets themselves, or one may be carried in one of the sheets and the other may be carried as a surface coating. Further, the reactive component for the colorless dye precursor may also be microencapsulated. Patents illustrative of many of the various kinds of systems which may be used in the production of manifold carbonless copying systems include, for example, U.S. Pat. No. 2,299,694 to Green, U.S. Pat. No. 2,712,507 to Green, U.S. Pat. No. 3,016,308 to Macaulay, U.S. Pat. No. 3,429,827 to Ruus and U.S. Pat. No. 3,720,534 to Macaulay et al.
As indicated above, a variety of arrangements exist for providing carbonless copying systems, the most common arrangement and the arrangement to which the present invention is particularly adaptable, is a system where microcapsules having a fill material including an initially colorless, chemically reactive, color forming dye precursor are coated on the back surfaces of each of the sheets of the manifold set and a dry coating containing a reactive component for the dye precursor is coated on the front surfaces of each of the sheets of the manifold set. One shortcoming of such carbonless copying systems is the tendency for the inadvertent and unintentional development of color on the CF coatings and the CB sheet. This inadvertent color development may be caused by the presence of free colorless dye precursor in the CB coatings due to incomplete encapsulation of the dye precursor or may be caused by accidental microcapsule rupture which can occur during halding, coating processes, printing processes and the like. In addition, the walls of the microcapsules may include faults or pores which allow the colorless dye precursor to thereby leak from the microcapsules. This free dye precursor often causes discoloration by contacting the CF component by passing through the base paper in the CFB sheets and also from sheet to sheet in a manifold set. Discoloration, which is variously referred to as blush, offset, bluing, ghosting, back print, etc. is highly objectionable and undesirable in a copying system.
A solution that has been proposed for these types of discoloration in carbonless copying systems is set forth in U.S. Pat. No. 4,000,087 to Maalouf. The patent teaches that microcapsules containing an initially colorless chemically reactive color forming dye precursor and a carrier therefor as the fill material and having generally continuous polyamide walls are rendered more resistant to inadvertent release and transfer of the fill material by incorporating into the fill material a resin selected from polystyrene resin and epoxy resin. While the solution for discoloration problems proposed by this patent has proved to be highly satisfactory, its applicability is limited to those microcapsules having a fill material compatible with a polystyrene or epoxy resin. In many carbonless copying systems utilizing microcapsules, the fill material of the microcapsules, especially the carriers for the colorless dye precursor, are not compatible with polystyrene and/or epoxy resins.
In addition, British Pat. No. 1,257,178 to Pilot Man-Nen-Hitsee Kabushik Kaisha proposes a method for making multilayer microcapsules such that the result microcapsules allegedly are not easily broken while being handled and avoid any percolation, moisture absorption, evaporation, oxidation, etc. of the encapsulated liquids and protect the contents in a chemically stabilized manner. The method of this patent is specifically directed to the manufacture of multilayer microcapsules contained liquid, using a hydrophobic material, a hydrophobic liquid, a hydrophilic liquid, one of the said materials containing two or more reactive functional groups so that the said hydrophobic material and the said hydrophilic material are capable of reacting interfacially at an interface between the hydrophobic liquid and the hydrophilic liquid thereby to form a high-polymer film insoluble in both of the said liquids. The method comprises dispersing either one of the said liquids, said one of the said liquids containing the hydrophobic or hydrophilic material according as the liquid is hydrophobic or hydrophilic, in the form of fine droplets in other liquid; causing a primary film to form on the surfaces of the said dispersed droplets thereby to prepare a suspension of microencapsulated liquid droplets; and adding the other said material to the said suspension, whereby the two said materials react together at any defects in the primary film to form a secondary film layer. A disadvantage of this method is that one of the materials to be reacted to form the secondary film layer must be included in the intended fill material.